The present invention relates to thread-forming fasteners such as self-tapping screws and to methods and apparatus for manufacturing the same.
The self-tapping screws are known having generally lobular leading ends with the thread formation in such ends having arcuate polygonal, in most instances triangular, pitch surface cross sections so as to require a minimum driving torque. Such screws may have a main shank portion of circular cross section for maximum holding strength, for example, as shown in U.S. Pat. No. 3,246,556, No. 3,180,126, and No. 3,461,470. However, a screw having a specified lobular pitch surface, cross section throughout its threaded extent has an advantage over a screw having a lobular pitch surface cross sections only on the leading end of the screw in that the former can be manufactured by rolling a similarly shaped blank between so-called flat conventional thread-rolling dies. See, for example, U.S. Pat. No. 3,195,156. Heretofore screws having a round body and a lobular end portion had to be made using special dies as in U.S. Pat. No. 3,246,556. Such special dies are not only expensive to manufacture but also have been found difficult to set up in the machine to give wholly satisfactory results. It has been recognized that a screw having the lobular pitch surface cross section only on the leading end and circular pitch surface cross sections on the main shank portion is a desirable form of screw for certain uses in requiring minimum driving torque while providing maximum holding strength.
In U.S. Pat. Nos. 3,180,126 and 3,461,470 it is suggested that screws with round body holding sections and lobular lead sections be made with conventional dies. However, in accordance with those two patents, the thread crests of the lobes on the tapered lead section are not fully formed. In U.S. Pat. Nos. 3,246,556 and 3,461,470, there is a relatively sharp transition between the lead section where the thread is lobular and the holding section where the thread is circular. In U.S. Pat. No. 3,180,126 thread-forming and sizing sections of lobular shape are used, and the method provides for a rocking about a fulcrum at the junction of the sizing and holding sections. As a result, it is not possible to form fully developed threads in the holding section at any significant distance from the fulcrum. This severely limits the length of holding section that can be produced.